Data input/out (I/O), which involves passing data from one computer component to another, is often a limiting factor or bottleneck in digital processing. For instance, an application running on a computer typically relies upon a device driver on a lower level (e.g., in the kernel mode) to feed data to it for processing. The performance of the application thus depends on how efficiently the driver can provide the data it needs for performing its task.
Conventionally, the application may receive data from the device driver in two modes. In the first mode, the application posts a request to receive data from the device driver if the requested data is currently unavailable. This request is handled asynchronously in that the application does not wait for the device driver to provide the requested data, and the requested data are delivered to the application at a late time when the data become available. In the second mode, the request for data is synchronously processed. In this case, the requested data are already placed in a queue and can be delivered immediately in response to the request. The asynchronous method of data delivery is generally less preferred because it incurs substantial processing and memory overhead, due to the need to allocate and initialize data structures for tracking the data request, as well as the overhead of asynchronous completion. The synchronous mode, in contrast, has no such overhead, but requires that the driver already has data queued for pickup in response to a system call by the application. Since there is no delay in delivering the requested data, the synchronous mode is also called “fast I/O” on the Microsoft Windows platform, although other platforms may use different terminology. Since the synchronous method has the advantages of significantly reduced overhead and immediate data delivery, it is desirable to increase its usage over that of the asynchronous method.